Several processes are available to separate suspended solids from liquid. Filtration and centrifugation are the most frequently applied solid separation methods. This invention, however, is useful in the dissolved air flotation method of solid separation. In a dissolved air flotation unit the air is dissolved under pressure into the liquid. For example, at 80 PSI pressure and 30.degree. C. about 7 times as much air can be dissolved than at atmospheric pressure. When the pressurized liquid is exposed to atmospheric pressure the excess air will come out of the pressurized liquid in the form of small air bubbles. The air bubbles collide with the suspended solid particles and become attached by absorption, entrapment and adhesion. The specific gravity of these particle-air bubble agglomerates is less than that of water so that they will rise to the surface forming a solid blanket. The clarified liquid is withdrawn from the tank as effluent.
For efficient operation of a dissolved air flotation unit, it is highly important that the bubbles coming out of the pressurized liquid are not too big. With smaller diameter bubbles, of course, more bubbles are formed for a given amount of air than with larger diameter bubbles thereby increasing the number of bubbles in a system for a given amount of air. Also, it has been found that when the bubbles become too large in diameter the small solids do not become attached to the bubbles and hence are not removed from the system. Most bubbles will range on the small side to about 30 microns in diameter and the major part of the bubbles should not exceed 100 microns.